Conventionally, a ceramic honeycomb structure or the like, which is a type of ceramic structure, is used for a wide range of applications, such as a catalyst carrier for automobile exhaust gas purification, a diesel particulate removal filter, or a heat storage body for a combustion device. A ceramic honeycomb structure (hereinafter simply referred to as a “honeycomb structure”) is manufactured by preparing a molding raw material (kneaded clay), extruding the kneaded clay into a desired honeycomb shape using an extruder, followed by raw cutting, drying and finish cutting, and then subjecting the material through a firing step of firing a honeycomb formed body at a high temperature.
In the honeycomb structure manufacturing method, the firing step of the honeycomb formed body can be largely divided into two main steps. For example, the firing step of the honeycomb formed body includes a debinder step of heating and removing organic matters, carbides, or the like included in the honeycomb formed body and a main firing step of sintering the debinded honeycomb formed body by heating the debinded honeycomb formed body at a high temperature.
There are two types of honeycomb structures. One is a honeycomb structure using oxide ceramic as a raw material, and the other is a honeycomb structure using non-oxide ceramic as a raw material. Although oxide ceramic can be main-fired in an air atmosphere, non-oxide ceramic needs to be main-fired in inert gas such as a non-oxidizing atmosphere such as argon gas. The present invention relates to the firing of a honeycomb structure made of non-oxide ceramic in particular.
In the main firing step, there are the case of using a continuous firing furnace and the case of using a single firing furnace. In particular, the continuous firing furnace is suitable for mass production of a honeycomb structure. In the case of using the continuous firing furnace as described above, a gas substituting chamber having a pair of airtight shutters capable of shutting off between a main firing furnace and the outside while maintaining airtightness is provided on the upstream side and the downstream side of the main firing furnace so as to perform the main firing step under inert gas. Therefore, before the honeycomb formed body is introduced into the main firing furnace and before the honeycomb fired body is taken out from the main firing furnace, it is possible to substitute an indoor space (closed space) of the gas substituting chamber with inert gas and it is possible to prevent the atmospheric air from entering a furnace space of the main firing furnace. As a result, in the furnace space of the main firing furnace, it is possible to perform the main firing step stably in the absence of oxygen. In the case of the single firing furnace (firing kiln) which does not include the gas substituting chamber, inert gas substitution is performed using a furnace space of the single firing furnace as a closed space before and after the main firing by high temperature heating.
An example of a conventional inert gas substituting method 100 will be mainly described with reference to FIG. 7. Here, an example of substituting the inert gas by a gas substituting chamber 101 installed on the upstream side of a main firing furnace (not illustrated) will be described in particular. The left side of FIG. 7 illustrates the gas substituting chamber 101 connected to the main firing furnace. A furnace-side airtight shutter 102a provided between the gas substituting chamber 101 and the furnace space of the main firing furnace is closed in advance, and airtightness between the furnace space and an indoor space 103 (corresponding to the closed space) of the gas substituting chamber 101 is maintained. Therefore, the flow of gas between the furnace space and the indoor space 103 is restricted. In this state, a honeycomb formed body is conveyed to the indoor space 103 from an inlet through which a one-side (outer-side) airtight shutter 102b is opened. In FIG. 7, the conveyed honeycomb formed body is not illustrated.
After the conveyance of the honeycomb formed body to the indoor space 103 is completed, the outer-side airtight shutter 102b is closed and the indoor space 103 of the gas substituting chamber 101 is closed by the pair of airtight shutters 102a and 102b (closing step S101). At this time, the indoor space 103 is filled with the atmosphere A, and the atmosphere A does not leak into the furnace space or the outside of the main firing furnace.
Next, the pressure in the indoor space 103 is reduced to a preset vacuum pressure by using a pressure reducing means (not illustrated) such as a vacuum pump, and a vacuum state is maintained (pressure reducing step S102). The set vacuum pressure is, for example, a pressure of about 5% with respect to the atmospheric pressure. Therefore, most of the indoor space 103 is a vacuum region V, and the atmosphere A slightly remains. FIG. 7 illustrates that the vacuum region V and the atmosphere A are separated and present in the indoor space 103, but this is for simplifying the illustration. In actuality, the vacuum region V and the atmosphere A are mixed and present therein (the same applies below).
After that, the operation of the pressure reducing means is stopped and inert gas 104 (argon gas or the like) is introduced into the indoor space 103 (pressure recovering step S103). At this time, the introduction of the inert gas 104 recovers pressure until the indoor space 103 has the same pressure as the atmospheric pressure, that is, up to 100% with respect to the atmospheric pressure. After that, the pressure reduction of the indoor space 103 (pressure reducing step S102) and the pressure recovery to the atmospheric pressure (pressure recovering step S103) are repeated several times, then the furnace-side airtight shutter 102a is opened, and the furnace space of the main firing furnace and the indoor space 103 are made to communicate with each other, and the honeycomb formed body is conveyed to the furnace space (conveying step S104).
As a result, it is possible to convey the honeycomb formed body while avoiding penetration of oxygen, nitrogen, or the like of the atmosphere into the furnace space of the firing furnace set in advance under inert gas. In particular, it is possible to avoid occurrence of problems such as influence on the firing of the honeycomb formed body due to penetration of oxygen having a specified oxygen concentration or more into the furnace space.
Even in the gas substituting chamber installed on the downstream side of the main firing furnace, the inert gas substituting method similar to the above is performed so as to prevent oxygen or the like from penetrating into the furnace space when the honeycomb fired body after the firing is taken out. In addition, even in the case of the single firing furnace not using the gas substituting chamber, the opening and closing between the furnace space (closed space) and the outside is performed by the airtight shutter provided at a furnace open end of the single firing furnace, the furnace space can be substituted with inert gas or the like by repeating the pressure reduction and the pressure recovery in the furnace space as in the gas substituting chamber. The firing is performed in the inert gas, and the furnace space after cooling in the furnace is substituted with the atmosphere.
A process of introducing inert gas or the like into a purge chamber (corresponding to a gas substituting chamber) after pressure reduction and recovering the internal pressures of the purge chamber and a heating chamber (corresponding to a main firing furnace) so as to be substantially equal to each other is disclosed in, for example, Patent Document 1.